How to use procedural light cookies (Blender Internal)
The easiest way would be to put a sub-divided mesh in front of your light, with a multi-slot material bank associated to that mesh. Then all you would have to do in each material slot is give each their own alpha value, then do a random face selection for each slot and assign it to the selection.

Just make sure that your material is equally RGB (R=G=B) for each slot, and you won't modify the color, only the tint/intensity.